1. Field of the Invention
The present invention relates to a method for forming a thin-film lithium-ion battery. More specifically, the present invention relates to a method for forming such batteries.
2. Discussion of the Related Art
Lithium-ion type batteries are formed of a stack of a cathode layer, an electrolyte layer, and an anode layer. During the battery charge, lithium ions travel from the cathode to the anode while, during the battery discharge, the lithium ions travel from the anode to the cathode.
The materials of the cathode, electrolyte, and anode layers are selected according to the voltage which is desired to be obtained across the battery and according to the desired application thereof. Especially, for data backup/retention applications in portable devices such as telephones or computers, the batteries are generally provided to supply a voltage smaller than 2 V. To obtain this voltage, a battery supplying a voltage ranging between 4 and 4.5 V, in association with a voltage step-down circuit may be used. Such a battery, for example, comprises a cobalt lithium oxide cathode (LiCoO2). A battery providing a voltage smaller than 3 V, for example comprising a cathode based on titanium oxysulfide (TiOS), may also be used. This last type of battery is considered herein.
FIG. 1 illustrates an example of a lithium-ion type battery formed in thin layers.
The battery is formed on a substrate 10 covered with a thin insulating layer 12, for example, made of silicon oxide, of silicon nitride, or of a stack of these materials. At the surface of insulating layer 12 is formed a cathode collector layer 14, for example, made of titanium. Layer 14 comprises a first portion, in a region A, at the surface of which the active stack of the battery is formed. Layer 14 comprises a second portion intended to receive an electric contact on its surface.
At the surface of layer 14, in active layer A, is formed a cathode layer 16.
As an example, layer 16 may be made of TiOS, a material capable of inserting lithium ions. At the surface of layer 16 is formed an electrolyte layer 18, for example, lithium phosphorus oxynitride (LiPON). In the shown example, layer 18 extends on one side of layer 16 to contact insulating layer 12. Layer 18 also extends on layer 14 on the contour of layer 16.
At the surface of electrolyte layer 18 is formed an anode layer 20, for example, made of silicon or germanium. Such amorphous materials have the specificity of becoming conductive under the influence of the lithium crossing them during the battery operation, the lithium transiently alloying with silicon or germanium.
At the surface of anode layer 20 is formed an anode collector layer 22, for example, made of titanium. Layer 22 extends, in the shown example, on layer 20 and on the extension of electrolyte layer 18 to reach the surface of insulating layer 12. The extension of layer 22 on insulating layer 12 enables forming anode contact. It should be noted that, in addition to their electric functions, titanium layers 14 and 20 also form barrier layers against the diffusion of lithium.
In the shown example, contact pads 24 and 26 are formed on the extensions of layers 14 and 22 formed at the surface of insulating layer 12. Generally, contact elements such as wires or solder bumps are then formed on contact pads 24 and 26, or directly on the extensions of layers 14 and 22. A passivation layer is then formed over the entire device. It should be noted that contact pad 26 may also be formed at the surface of the stack of layers 16, 18, 20, and 22 if the extension of layer 22 is not provided.
A structure such as that in FIG. 1, with layer 16 made of TiOS, associated with lithium ions, is generally formed as follows. Cathode collector layer 14 is formed at the surface of insulating layer 12 by means of an adapted mask or by etching of a full plate layer. Then, a TiOS layer is formed at the surface of the cathode collector layer. This layer is transformed into an LiTiOS layer to become cathode layer 16 by direct thermal metal evaporation of lithium. Then, by means of an adapted mask or by etching, layers 18, 20, and 22 are formed at the surface of cathode layer 16.
The insertion of lithium by the above method does not enable to form a cathode layer 16 of good quality. This lack of quality is considered as resulting from the fact that, during this step and during the next heating steps, for example to form solder bumps, sulfur compounds of lithium such as Li2S, Li2SO3, or Li2SO4 form at the surface and across the volume of the TiOS layer. Such compounds decrease the quality of the cathode layer, and thus the battery performance.
There thus is a need for a method for forming a thin-film lithium-ion type battery comprising an LiTiOS cathode layer of improved quality.